1. Field of the Invention
The present invention relates to a method for fabricating a liquid crystal display, and more particularly, to a method for fabricating a liquid crystal display (LCD) device using two masks.
2. Description of the Related Art
As interest in image display devices and demand for portable information device increases, a thin film type Flat Panel Display (FPD) devices have been developed and replaced traditional Cathode Ray Tubes (CRT) type display devices. In particular, a liquid crystal display (LCD) device having the characteristic of an optical anisotropy has replaced the CRT. Further, the liquid crystal display device has been used in notebook computers, desktop monitors, or the like because it has an excellent resolution, color rendering capability and picture quality.
A liquid crystal display device includes a color filter substrate, which is a first substrate, an array substrate, which is a second substrate, and a liquid crystal material formed between the two substrates. A thin film transistor, which uses amorphous silicon or polycrystalline silicon as a channel layer, is used as a switching device of the liquid crystal display device. A process for fabricating the liquid crystal display device usually requires a plurality of photo-mask processes in fabricating the array substrate including the thin film transistor.
FIG. 1 is a plan view showing a unit pixel of an array substrate of a liquid crystal display device of the related art. In a typical liquid crystal display device, the ‘N’ number of gate lines and the ‘M’ number of data lines cross each other, forming the ‘N x M’ number of pixels. For the purpose of simplicity, only one pixel is presented in the FIG. 1. As shown in FIG. 1, the array substrate 10 includes: a pixel electrode 18 formed at a pixel region; gate lines 16 and data lines 17 disposed vertically and horizontally on the substrate 10; and a thin film transistor used as a switching device, which is formed at a region adjacent to where the gates lines 16 and the data lines 17 cross each other.
The thin film transistor includes: a gate electrode 21 connected to the gate line 16; a source electrode 22 connected to the data line 17; and a drain electrode 23 connected to the pixel electrode 18. Also, the thin film transistor includes: a first insulating layer (not shown) and a second insulating layer (not shown) for insulating the gate electrode 21 and source/drain electrodes 22 and 23; and an active layer 24 for forming a conductive channel between the source electrode 22 and the drain electrode 23 by a gate voltage supplied to the gate electrode 21. The source electrode 22 is electrically connected to a source region of the active layer 24 and the drain electrode 23 is electrically connected to a drain region of the active layer 24 through first contact holes 40a formed in the first and second insulating layers. Since a third insulating layer (not shown) has a second contact hole 40b exposing the drain electrode 23, the drain electrode 23 and the pixel electrode 18 are electrically connected to each other through the second contact hole 40b. 
FIGS. 2A to 2G are cross-sectional views along the line I-I of the liquid crystal display device shown in FIG. 1 showing fabrication processes according to the related art. As shown in FIG. 2A, the active layer 24 of polysilcon is formed on a transparent substrate 10, such as a glass, by using a photolithography process.
Next, as shown in FIG. 2B, a first insulating layer 15a is deposited over the entire surface of the substrate 10 on which the active layer 24 is formed and a conductive metal layer 30 are deposited on the first insulating layer 15a. 
Next, as shown in FIG. 2C, a gate electrode 21 is formed on the active layer 24 with the first insulating layer 15a interposed therebetween by patterning the conductive metal layer 30 using a photolithography process. Thereafter, source/drain regions 24S and 24D are formed by injecting N+ or P+ high density impurity ions into side regions of the active layer 24 using the gate electrode 21 as a mask. The source/drain regions 24S and 24D are formed for ohmic contacts of the active layer to the source/drain electrodes.
Next, as shown in FIG. 2D, after a second insulating layer 15b as an interlayer is deposited over the entire surface of the substrate 10 on which the gate electrode 21 is formed, the first contact holes 40a exposing the source/drain regions 24S and 24D are formed through the first insulating layer 15a and the second insulating layer 15b through photolithography.
Thereafter, as shown in FIG. 2E, after a conductive metal is deposited over the entire surface of the substrate 10, the source electrode 22 connected to the source region 24a and the drain electrode 23 connected to the drain region 24b are formed through the first contact holes 40a. At this time, a part of the conductive metal constructing the source electrode 22 is extended to be connected to the data line 17.
Next, as shown in FIG. 2F, a third insulating layer 15c, an organic insulating layer, such as Acryl, is deposited over the entire surface of the substrate 10, and the second contact hole 40b is formed to expose the drain electrode 23 by photolithography.
Finally, as shown in FIG. 2G, transparent conductive materials, such as Indium Tin Oxide (ITO), are deposited over the entire surface of the substrate 10 and into the second contact hole 40b. The pixel electrode 18 is connected to the drain electrode 23 through the second contact hole 40b and patterned by using photolithography on the transparent conductive materials. More specifically, the photolithography forms an active pattern, a gate electrode, a first contact hole, source/drain electrodes, a second contact hole and a pixel electrode, respectively, when fabricating a liquid crystal display device including a polysilicon thin film transistor.
Photolithography refers to a series of processes in which desired patterns are formed by transcribing a pattern drawn on a mask to a substrate on which a thin film is deposited, and includes a plurality of processes, such as photoresist solution application, exposure, development, or the like. As a result, a plurality of photolithography processes lower production yield and are prone to cause defects in the thin film transistor. Since a mask used for photolithography is very expensive, if many number of masks are used to form LCD device, ion costs for the liquid crystal display device are also increased. Thus, productivity can be increased, if the usual number of photo-mask steps is decreased.